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Brain 'hunger pathways' pinpointed

The brain circuitry that influences how much food a person will eat – whether they feel starving or full – has been revealed by a new imaging study. The results may help target new treatments against obesity, say researchers.

Rachel Batterham at University College London, UK, and her colleagues have previously shown that a hormone called peptide YY or PYY, which is released by the gut in proportion how many calories we eat, is a powerful appetite suppressant. Previous experiments show that treating normal and obese subjects with intravenous PYY decreases food intake by up to 30%.

Batterham’s team used functional magnetic resonance imaging (fMRI) to investigate how PYY affects the brain. They scanned each of eight subjects twice, once while they were on an intravenous drip of PYY, mimicking its release after a meal, and once while just receiving a saline solution. All the subjects had fasted for 14 hours prior to the scans.

Half an hour after they left the scanner, Batterham dished out an all-you-can-eat buffet of each subjects’ favourite meals, which included spaghetti Bolognese and macaroni cheese.

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As expected, those who received PYY ate less – on average 25% fewer calories. The fMRI scans showed that PYY not only lit up the hypothalamus – the main hub for controlling metabolism, – but also increased activity in higher processing areas of the brain that are associated with reward and pleasure, notably the orbital frontal cortex (OFC). “I absolutely wasn’t expecting it to affect the reward circuit,” says Batterham.

Gastric bypass

What is more, changes in the activity of these two areas could accurately predict how much food the subjects would eat at their buffet lunch. For those on the saline drip, activity in the hypothalamus predicted how many calories they would consume. But for those subjects who received PYY, it was the pleasure centre of the brain that determined calorie intake.

Batterham hopes to repeat the study on both underweight and overweight subjects. Previous research has shown that obese individuals have lower levels of PYY and release less of the hormone after a meal compared with normal weight people. Gastric bypass surgery has also been show to boost levels of PYY. “We still have a lot to learn about PYY,” she says.

Neurologist and obesity researcher Clifford Saper at Harvard Medical School in Boston, says the study is interesting, as it is the first to explore cognitive aspects of eating behaviour with respect to an appetite hormone. What is more, he says, understanding the higher level drives behind eating behaviours is key for treating obesity.

“We humans are surrounded by all these tempting foods that taste good, and many people eat way past the point of metabolic satiety,” Saper says.

Michael Schwartz at the University of Washington in Seattle, who also studies the relationship between brain activity and eating, says&colon; “I think it’s an impressive body of work and it does push the field forward.”